The use of selective herbicides for controlling specific weeds or plants in crops has become almost a universal practice. The market for these herbicides approaches a billion dollars annually. Even with this extensive use, weed control remains a significant and costly problem for the farmer.
Present day herbicides used singly or in so-called tank mixes require careful management to be effective. Time and method of application and stage of weed plant development are critical to getting good weed control with herbicides. Application of large amounts of preemergence herbicides can result in a commitment to grow the same crop in subsequent years because of chemical persistence in the soil which prevents rotation with a crop sensitive to that herbicide. Furthermore, some weed species are simply resistant to the available herbicides. Therefore, the development of effective herbicides increases in importance every year, especially as other weeds are controlled and thus reduce interplant competition.
Weed control in maize is currently accomplished by soil application of herbicides that are applied before the crop emerges and prior to the observation of a weed problem. The preemergence herbicides currently used adequately control most dicot and monocot (grass) weeds in maize. However, annual grass weeds such as wild proso millet and wooly cupgrass and perennial grass weeds commonly escape preemergence weed control. Preemergence herbicides require rainfall for activation, and under low rainfall conditions they fail to control grass weeds in maize. Furthermore, some preemergence herbicides persist in the soil and several have been detected as groundwater contaminants. The options for controlling these escape grass weeds are very limited.
A postemergence herbicide for grass weed control in maize would be very beneficial. An attractive alternative to developing new herbicides to combat this weed control problem in maize and/or to decrease the amount of herbicide carryover and groundwater contamination in maize fields from the existing herbicides is to develop maize hybrids or varieties that are tolerant to other existing herbicides that normally kill all monocot (grass) species. The herbicide POAST(trademark) (BASF Corp., Parsippany, N.J.), the active ingredient of which is sethoxydim, kills most grasses, and is applied at lower rates than many preemergence herbicides. POAST(trademark) is nonpersistent in the environment and therefore does not represent a groundwater contamination threat. POAST(trademark) tolerant maize would provide the producer with increased weed management flexibility because POAST(trademark) could be applied when a grass weed problem was detected without risk of damage to the crop and only to the areas with a weed problem. Therefore, postemergence control of local weed problems would further decrease the amount of herbicide applied compared to existing preemergence weed control strategies.
The sensitivity of maize to some herbicides is the result of the presence of herbicide sensitive forms of acetyl CoA carboxylase (ACCase) in those plants. ACCase is an enzyme involved in many important metabolic pathways in plant, animal and bacterial cells. Structurally, ACCases are biotinylated and are quite large enzymes consisting of one or more subunits. For example, most ACCases of animals, higher plants, and yeast are dimers of 420 to 700 kD native MW and contain subunits of 200 to 280 kD.
Two forms of ACCase, termed ACCase I and ACCase II, can be isolated from maize. These forms differ in their size, charge, cellular location, immunoreactivity with ACCase I antiserum, and sensitivity to herbicide inhibition. The predominate form, ACCase I, is plastid localized and is sensitive to herbicide inhibition.
Four ACCase genes have been identified in maize by Southern blot analyses (Lutz et al., Maize Genetics Conference (1995)). Restriction fragment length polymorphism (RFLP) analyses mapped one ACCase gene, termed Acc1, to chromosome 2 between umc131 and umc2b (FIG. 1) in recombinant inbred lines from Txc3x97303xc3x97C059 (Egli et al., Maize Genetics Coop. Newsletter, 68, 92 (1994)). Mutations in the Acc1 gene can result in ACCase that is resistant or tolerant to herbicide inactivation ((Marshall et al., Theor. Appl. Genet., 83,435 (1992); Egli et al., Plant Physiol., 101, 499 (1993); Egli et al., MNL, 66, 94 (1992)). However, the R0 plants which were regenerated from tissue cultures selected for resistance (or tolerance) to herbicides, by virtue of mutations in Acc1, exhibit only partial herbicide resistance, i.e., the symptoms induced by herbicide exposure are not prevented (see U.S. Pat. No. 5,162,202).
Caffrey et al. (Maize Genetics Coop. Newsletter, 69, 3 (1995)) disclose that RFLP analyses of recombinant inbreds derived from the crosses Txc3x97303xc3x97CO159 and T232xc3x97CM37 showed that one ACCase gene maps to chromosome 2 between umc131 and uox while another ACCase gene maps to chromosome 10 between ncsu2 and umc155 (FIG. 2). Caffrey et al. further disclose that the ACCase gene on chromosome 10 appears to correspond to an herbicide resistance locus described by Van Dee et al. (Agro. Abs., page 198 (1992)). The authors propose that the ACCase gene on chromosome 10 encodes an herbicide sensitive ACCase that is localized to the plastid while the ACCase gene on chromosome 2 encodes an herbicide resistant ACCase that is non-plastid localized.
Thus, there is a need for a method to prepare a maize plant with resistance or tolerance to herbicides.
The invention provides a method to prepare maize (xe2x80x9ccornxe2x80x9d or Zea mays L.) plants with resistance or tolerance to cyclohexanedione or aryloxyphenoxypropionate herbicides, i.e., maize plants with high tolerance to field application rates of herbicide. The herbicide resistance or tolerance is the result of the plants having at least one copy of each of two herbicide resistant or tolerant acetyl CoA carboxylase (ACCase) genes, one of which is encoded on chromosome 2 (Acc1) and the other of which is encoded on chromosome 10 (Acc2), i.e., the plant is a double heterozygous mutant (e.g., Acc1-S2/+;+/Acc2-S5). In contrast, when a plant has only one copy of either of the herbicide resistant ACCase genes, e.g., Acc1-S2/+;+/+ or +/+;Acc2-S5/+, the plant exhibits only partial tolerance to the herbicide, i.e., the expression of the herbicide resistant ACCase gene does not prevent symptoms of herbicide damage under standard field-application rates of herbicides.
In commercial corn breeding practices, the double heterozygous genotype can be a hybrid variety planted by farmers. To prepare the double heterozygote, a homozygous double mutant parent line (e.g., Acc1-S2/Acc1-S2;Acc2-S5/Acc2-S5) is crossed to a normal (nonmutant) susceptible parent line. Thus, corn breeders can maintain fewer homozygous double mutant parent lines than would be needed with a system in which both parents must be homozygous for a single mutant gene to obtain a fully herbicide resistant maize plant.
Thus, the invention provides a method of imparting cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance to a corn plant. The method comprises crossing a first corn plant with a second corn plant so as to yield progeny plants. The first plant is homozygous for an allele of Acc1 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance. The second plant is homozygous for an allele of Acc2 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance. The progeny plant is heterozygous for the Acc1 allele which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance and heterozygous for the Acc2 allele which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance. A preferred embodiment of the invention is a method of imparting herbicide tolerance to a hybrid plant.
The invention also provides a method of imparting tolerance to a corn plant to an agent which inhibits acetyl CoA carboxylase, wherein the agent is selected from the group consisting of 3-(2,4-dichlorophenyl)-perhydroindolizine-2,4-dione (Babczinski et al., Pesti. Sci., 33, 455 (1991)), 3-isopropyl-6-(N-[2,2-dimethylpropyl]-acetamido-1,3,5-triazine-2,4-(1H,3H)dione (Walker et al., Phytochem., 29, 3743 (1990)), soraphen A (Vahlenesiock et al., Curr. Genet., 25 93 (1994)), and structural and/or functional analogs thereof. The method comprises crossing a first corn plant, which is homozygous for an allele of Acc1 which imparts tolerance to the agent, with a second corn plant, which is homozygous for an allele of Acc2 which imparts tolerance to the agent, so as to yield progeny plants. The progeny plant is heterozygous for the Acc1 allele which imparts agent tolerance and heterozygous for the Acc2 allele which imparts agent tolerance.
Also provided is a method to prepare an herbicide resistant or tolerant corn plant. The method comprises crossing a first corn plant which comprises at least one herbicide resistant allele with a second corn plant which comprises at least one herbicide resistant allele which is not allelic to the herbicide resistant allele in the first plant, to yield a progeny plant which is a heterozygote for each allele.
The invention further provides a method of imparting cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance to a corn plant. The method comprises self pollinating a corn plant which comprises (i) an allele of Acc1 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance and (ii) an allele of Acc2 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance, so as to yield a progeny plant. Then a progeny plant is identified that is homozygous for the allele of Acc1 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance and is homozygous for the allele of Acc2 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance.
Also provided is a method of preparing a plant which is a double heterozygote for alleles of Acc1 and Acc2 which impart cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance. The method comprises crossing a first corn plant with a second corn plant so as to yield progeny plants. The first plant is homozygous for an allele of Acc1 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance and the second plant is homozygous for an allele of Acc2 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance. The progeny plant is heterozygous for the Acc1 allele which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance and heterozygous for the Acc2 allele which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance.
The invention also provides a method of preparing a plant which is a double homozygote for alleles of Acc1 and Acc2 which impart cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance. The method comprises self pollinating a corn plant which comprises (i) an allele of Acc1 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance and (ii) an allele of Acc2 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance, so as to yield a progeny plant. A progeny plant is identified that is homozygous for the allele of Acc1 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance and is homozygous for the allele of Acc2 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance.
Also provided are progeny and seed derived from the plants prepared by the methods described herein.
Yet another embodiment of the invention is an inbred or hybrid cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerant Zea mays plant. The genome of the inbred or hybrid plant of the invention is homozygous for an allele of Acc1 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance, and is homozygous for an allele of Acc2 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance.
A further embodiment of the invention is an inbred or hybrid cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerant Zea mays plant, the genome of which is heterozygous for an allele of Acc1 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance, and is heterozygous for an allele of Acc2 which imparts cyclohexanedione or aryloxyphenoxypropanoic acid herbicide tolerance.
Progeny and seed derived from the inbred plants of the invention are also provided.
As used herein, the term xe2x80x9ccyclohexanedione herbicidexe2x80x9d includes, but is not limited to, 1,3-cyclohexanediones which exhibit general and selective herbicidal activity against plants. One such cyclohexanedione is sethoxydim {2-[1-(ethoxyimino)-butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one}. Sethoxydim is commercially available from BASF (Parsippany, N.J.) under the designation POAST(trademark).
Other herbicidal cyclohexanediones falling with in the scope of the invention include clethodim, (E,E)-(xc2x1)-2-[1-[[(3-chloro-2-propenyl)oxy]imino]propyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one; available as SELECT(trademark) from Chevron Chemical (Valent) (Fresno, Calif.); cloproxydim, (E,E)-2-[1-[[(3-chloro-2-propenyl)oxy]imino]butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one; available as SELECTONE(trademark) from Chevron Chemical (Valent) (Fresno, Calif.); and tralkoxydim, 2-[1-(ethoxyimino)propyl]-3-hydroxy-5-mesitylcyclohex-2-enone, available as GRASP(trademark) from Dow Chemical USA (Midland, Mich.), as well as other cyclohexanedione herbicides that are structurally related to the compounds described hereinabove.
As used herein, the term xe2x80x9caryloxyphenoxypropanoic acid herbicidexe2x80x9d includes aryloxyphenoxypropanoic acids which exhibit general and selective herbicidal activity against plants. Such herbicides include, but are not limited to compounds wherein the aryloxy group may be phenoxy, pyridinyloxy or quinoxalinyl. One such herbicidal aryloxyphenoxypropanoic acid is haloxyfop, {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]-propanoic acid}, which is available as VERDICT(trademark) from Dow Chemical USA (Midland, Mich.). Another is diclofop, {(xc2x1)-2-[4-(2,4-dichlorophenoxy)-phenoxy]propanoic acid}, available as HOELON(trademark) from Hoechst-Roussel Agri-Vet Company (Somerville, N.J.). Other aryloxyphenoxypropanoic acid herbicides within the scope of the invention include fenoxyaprop, (xc2x1)-2-[4-[(6-chloro-2-benzoxazolyl)oxy]phenoxy]propanoic acid; available as WHIP(trademark) from Hoechst-Roussel Agri-Vet Company (Somerville, N.J.); fluazifop, (xc2x1)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid; available as FUSILADE(trademark) from ICI Americas (Wilmington, Del.); fluazifop-P, (R)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid; available as FUSILADE 2000(trademark) from ICI Americas (Wilmington, Del.); and quizalofop, (xc2x1)-2-[4[(6-chloro-2-quinoxalinyl)-oxy]phenoxy]propanoic acid; available as ASSURE(trademark) from E. I. DuPont de Nemours (Wilmington, Del.), as well as other herbicidal compounds which are structurally related to the compounds described hereinabove.
As used herein, a plant that is xe2x80x9cresistant or tolerant to inhibition by a herbicide or agentxe2x80x9d is a plant that grows in an amount of herbicide or agent that normally inhibits growth of a corresponding susceptible plant, as determined by methodologies known to the art. For example, a maize plant of the invention (e.g., Acc1-S2/Acc1-S2;Acc2-S5/Acc2-S5 or Acc1-S2/+;Acc2-S5/+) grows in an amount of cyclohexanedione that inhibits the growth of a corresponding susceptible maize plant (e.g., +/+;+/+, Acc1-S2/+;+/+ or +/+;Acc2-S5/+). In the alternative, the herbicide resistance of a homozygous backcross converted inbred plant of the invention (Inbred A (Acc1-S2/Acc1-S2;Acc2-S5/Acc2-S5) is compared to the herbicide resistance of a recurrent inbred susceptible plant (Inbred A (+/+;+/+)). A homozygous backcross converted inbred plant of the invention is a plant which has been repeatedly crossed to the recurrent inbred parent until the backcross converted inbred plant is substantially isogenic with the recurrent inbred parent except at Acc1 and Acc2 loci, and is then self-pollinated (selfed) at least once.
As used herein, xe2x80x9csubstantially isogenicxe2x80x9d means that the genomic DNA content of a homozygous backcross converted inbred plant is at least about 92%, preferably at least about 98%, and most preferably at least about 99%, identical to the genomic DNA content of a recurrent inbred parent of the backcross converted inbred plant.
Exemplary susceptible maize lines, e.g., lines which are sensitive to growth inhibition by cyclohexanedione or aryloxyphenoxypropanoic acid herbicides include, but are not limited to, A188, A641, A619, B73 and Zea mays, var. PI 3140. Exemplary maize lines which are a source of either Acc1 or Acc2 herbicide resistance alleles include, but are not limited to, Zea mays, var. DK 592SR, Zea mays, var. DK 404SR, 4400SR and 7800SR.